1. Field of the Invention
The present invention relates generally to a method of treating chemical-containing liquors from pulping operations to remove the chemicals contained therein, and to a method of recovering said chemicals. More particularly the invention relates to passing liquors, containing chemicals produced during pulping operations, through one or more units containing activated carbon to remove and separate the chemicals present in said liquors. The chemicals are subsequently recovered from the activated carbon units by introducing a solvent into the unit.
2. Description of the Prior Art
In the preparation of wood pulp useful in paper-making operations wood chips are treated, either chemically or mechanically, to separate the cellulosic fibers. In the sulfate, or kraft, pulping process wood chips are treated with a cooking liquor containing sodium hydroxide, sodium sulfide and sodium carbonate. In the sulfite pulping process wood chips are treated with a cooking liquor containing sulfur dioxide, sulfurous acid and bisulfite, usually as sodium, calcium, magnesium or ammonium bisulfite. The chemicals attack the non-fibrous materials present in the wood chips, reacting with the lignin to form water-soluble compounds, thereby allowing separation of the fibrous, or cellulosic, portion of the chips.
The treatment of the wood chips normally takes place in a digester over an extended period of time and at an elevated temperature. The exact time, temperature and pressure depend, to a considerable extent, on the species of wood and the amount of chemicals employed. To maintain a constant temperature and pressure during the chemical treatment, vapors are periodically released from the digester. Also, before the pulp is discharged from the digester the temperature and pressure are lowered by releasing additional vapors. All of these vapors are collected in high pressure accumulators and are commonly referred to as digester relief gases. The pulp and spent pulping liquor can be removed from the digester by being blown out at low pressure or dumped. During their removal additional vapors are released which are commonly referred to as digester blow gases. The digester blow gases can be condensed and the condensate collected in a hot water accumulator where it is allowed to cool and overflow into a drain. This overflow is commonly referred to as hot water accumulator overflow. The digester relief gases, digester blow gases, spent pulping liquor and hot water accumulator overflow contain chemical by-products of the pulping operation including, for example, sulfur dioxide, methanol, acetone, acetaldehyde, ethanol, furfural, p-cymene, acetic acid, dimethyl disulfide, dimethyl sulfide, methyl mercaptan, crude turpentine and hydrogen sulfide. The type and amount of chemical in any given sample depends on many factors including the species of wood, treatment chemicals and cooking conditions employed. After the fibrous pulp is separated, the cooking liquor, now commonly referred to either as spent kraft pulping liquor (or black liquor) or spent sulfite pulping liquor, depending on the pulping process employed, must be disposed of. In disposing of these liquors as well as the condensed gases described above, both economic and environmental factors must be considered. Much effort has been expended in recent years to find an economical and practical means for disposing of, for example, the spent kraft pulping liquor. The most commonly employed treatment is the evaporation of the liquors to recover the sodium salts present therein for reuse in subsequent pulping operations. The evaporator condensates and vent gases from this treatment are also known to contain various chemicals produced during the pulping operation.
The recovery of saleable by-products from the liquors and condensates produced during kraft pulping operations has been suggested. The recovery of crude turpentine, or pinene, from the relief gases of kraft digesters is well known. See for example, Casey, Pulp and Paper, Vol. 1, second edition, Interscience Publishers, Inc., New York, (1960) pages 284-285. However, no commercially acceptable process for recovering all of these chemicals has heretofore been available. Most of the prior art processes recovered only one of the chemicals and the purity of the recovered chemicals was poor requiring further processing to obtain saleable products.
Much effort has also been expended to find an economical and practical means of disposing of spent sulfite pulping liquor. Use of the spent sulfite pulping liquor as a road binder has been suggested as a solution to the problem. The recovery of saleable by-products has also been suggested. However, the most commonly used treatment is evaporation and subsequent burning of the concentrated liquor to produce steam and power for use in subsequent pulping operations and to recover chemicals, such as sulfur dioxide and sulfites, also for use in subsequent pulping operations.
Spent sulfite pulping liquors are known to contain, besides sulfur dioxide, organic chemicals such as methanol, ethanol, acetic acid, furfural and p-cymene. These organic compounds are formed as by-products of the pulping process. It has previously been suggested that these chemicals be recovered from the spent sulfite pulping liquor. However, no commercially acceptable process for recovering these chemicals has heretofore been available. Most of the prior art processes recovered only one of these chemicals and to recover all of them would require a series of treatments. Also, the purity of the organic chemicals recovered by the previously known processes has been poor and further processing was often required to obtain saleable products.
In U.S. Pat. No. 1,838,109, issued to Richter, furfural was obtained as a by-product in the preparation of wood pulp by the sulfite pulping process. The furfural was recovered by heating the spent pulping liquor, at an acid pH, to a temperature sufficient to vaporize the furfural and then condensing the vaporized furfural. This process required temperatures in excess of 250.degree. F., which often resulted in decomposition of the furfural. Also, the acid pH required was achieved by the use of sulfur dioxide gas, the presence of which has now been found to have a deleterious effect on the furfural produced. Other chemicals in the spent sulfite pulping liquor were not recoverable by this process.
In U.S. Pat. No. 1,223,158, issued to Enger, the "hydrocarbon cymol" was isolated from digester relief gases obtained from a sulfite pulping process by allowing the condensed gases to stand in a plurality of tanks during which time the cymol came to the top and could be separated. The liquor was then returned to the digester. The purpose of this process was not to recover the organic chemical but rather to purify the condensate so it could be reused in subsequent pulping operations.
In U.S. Pat. No. 1,833,955 issued to Richter, alcohol (ethanol) was obtained by fermentation of the sugars contained in partially evaporated spent sulfite pulping liquor.
However, there has not heretofore been available a method for recovering and separating chemicals produced during both sulfite and kraft pulping operations.
Activated carbon has been used for water purification, including the treatment of pulp and paper mill effluents. When working with these effluents the purpose of the activated carbon treatment has included the removal of color and odor and the reduction of the chemical oxygen demand and the biological oxygen demand of the effluents. However, it has not heretofore been suggested to utilize activated carbon treatments to remove and separate, in a useable form, the chemicals present in pulp mill effluents.
A commercially acceptable activated carbon separation process has not been available due, at least in part, to the absence of a satisfactory method for regenerating the activated carbon. Previously available regeneration techniques included the use of regeneration furnaces, wet air oxidation and caustic soda washing. These techniques are time consuming and expensive, and result in at least partial destruction of the chemicals removed from the carbon.